Screw-conveyor centrifuges consist of a centrifuge bowl that is supported rotatably about its longitudinal axis and that encloses a coaxially arranged screw conveyor turning at an unequal rotation speed (leading or lagging). Screw helices are attached to a screw barrel and means are provided for feeding the mixture for separation into the centrifuge bowl along the axis of the centrifuge bowl. Openings are provided at opposite ends of the centrifuge bowl for the discharge of the separated light and heavy substances. The radial distance from the centrifuge bowl shaft axis to said openings is less than the distance from the centrifuge bowl shaft axis to the centrifuge bowl wall, so that during the operation of the screw-conveyor centrifuge a liquid ring forms inside the centrifuge bowl under the action of centrifugal force.
In the separation of liquid-solid mixtures in screw-conveyor centrifuges, under the action of the centrifugal force generated by the rotation of the screw-conveyor centrifuge, the centrifugal force is a multiple of the force of gravitation, and there is formed inside the centrifuge bowl a ring of the liquid-solid mixture being separated. This ring consists primarily of already separated liquid, the inside diameter of which is determined by the radial arrangement of the discharge openings for the light substances (the separated liquid).
The separation into substances of greater and of lesser specific gravity takes place in this liquid ring under the action of the centrifugal force, the substances of greater specific gravity (the solids) arranging themselves at the outer margin of the ring, thus at the centrifuge bowl wall, and the substances of lesser specific gravity (the liquid) disposing themselves layerwise thereover in the inner portion of the ring. By the screw helices of the screw conveyor, which, depending on the rotation direction of the centrifuge, are set in rotation in the same rotation direction, leading or lagging relative to the rotation speed of the centrifuge bowl, the heavy substances are picked up and transported to one end of the centrifuge where they exit the centrifuge through appropriate openings. The light substances flow to the opposite end of the centrifuge, to the discharge openings arranged there, flowing at least partially oppositely to the conveyance direction of the screw conveyor by means of channels formed from the screw helices of the screw conveyor.
When the centrifuge is turned off, the rotation speed of the centrifuge diminishes to a stop and the centrifugal force also diminishes therewith. In this process, a rotation speed is reached at which the centrifugal force becomes less than the force of gravitation, with the consequence that the liquid ring formed in the centrifuge "collapses" and the level inside the centrifuge reaches over the discharge openings for the heavy substances (solids), so that the light substances also exit surgewise at these openings, even if only for a brief time. By this means, not only is the separation result degraded, but also process disturbances are to be anticipated in the further processing of the heavy substances.